There are many forms for display apparatus employing a raster scan, such as TV receivers and picture monitors. Such type of apparatus is referred to herein as a raster scan display apparatus. For evaluating a raster scan display apparatus, it is convenient to display a test pattern, such as vertical lines, horizontal lines or a combination of vertical and horizontal lines, on a screen of the display apparatus. The displayed test pattern is compared with a standard graticule attached to the screen to measure linearity, pincushion distortion and also convergence.
Different horizontal frequencies, scan lines per field and vertical frequencies are used in different TV systems. Each different system needs a proper test pattern generator which generates a proper test pattern for the system. If the test pattern generator of one system is employed with a raster scan display apparatus of another system, the test pattern may not be displayed because of a synchronization problem. With conventional techniques, even if the display apparatus can accept the output signal from the test pattern generator, the number and spatial width of the displayed vertical lines may change, and the number of the displayed horizontal lines may change. Thus, using conventional techniques, each TV system needs its own test pattern generator in order to provide a predetermined test pattern for evaluation. In order to search for scan rates which display interactions such as a Moire pattern, or to calibrate displays at nonstandard scan rates, it is necessary to provide a test pattern signal having a continuum of horizontal frequencies, scan lines per field, and vertical frequencies. However, conventional test pattern generators cannot generate a test pattern at variable frequencies without changing the displayed pattern.
The present invention may be used to provide a test pattern generator which generates a test pattern signal which can be employed in a TV system with continuously variable vertical and horizontal frequencies and scan lines per field. The invention may also be used to provide a test pattern generator for displaying a substantially constant test pattern on a raster scan display apparatus regardless of the vertical and horizontal scanning frequencies and the number of scan lines per field.